Since the first introduction of polymer electrolytes as a new class of solid electrolyte for energy storage applications, studies of all solid polymer electrolytes with ionic conductivity of 10−5–10−2 S/cm at ambient temperatures have received much attention owing to the potential applications in various electronic devices. Most of the previous efforts were based on the poly(oxyethylene) complexes with inorganic salts such as LiClO4, LiSO3CF3 and most recently, LiN(SO2CF3)2, these systems are often denoted as binary electrolytes as both anion and cation contribute to the ion transportation. To achieve the practical applications of polymer electrolytes, the polymer has to satisfy several rigorous requirements: (1) bear strong ion coordinating sites to solvate inorganic salts, (2) must be amorphous with substantial segmental flexibility, (3) have durable mechanical and electrochemical stability for specific application environments. Numerous attempts have been tried to increase the conductivity by making PEO derived polymers, such as block copolymers, comb-branch polymers containing methylated poly(ethylene glycol) side chains, polymer networks and adding plasticizer or additives to break down the crystalline phase, which is detrimental to the transportaton of charge carriers. In all these cases, however, the essential problem related to the efficiency of rechargeable polymer lithium batteries, i.e., polarization and very low Li+ transference number is inevitable with a binary salt electrolyte. It is, therefore, desirable to properly design and synthesize polymers with the anion attached covalently to the polymer chain. The instant invention solves the aforementioned problems. Compared with binary systems, single-ion conductors show constant dc conductivity during dc polarization and shall have Li+ cation transfer number of 1. Normally, single-ion conductors have much lower conductivity than binary salt electrolytes under the same conditions, in the range of 10−8 to 10−6 S cm−1 at 25° C. for alkali metal cations, due to the ion paring to the immobile anion. It is therefore necessary to develop new materials with improved conductivity, e.g., 10−4 S cm−1 at room temperature, if they are to be used for lithium rechargeable batteries.
Hyper-comb-branched polymer conjugates are known in the art, for example U.S. Pat. No. 5,919,442 to Yin et al. Therein are described a class of hyper comb-branched polymers conjugates with carrier materials. This reference is incorporated herein by reference in its entirety.
One major limitation for comb branched polymers is the lack of mechanical strength to form free standing films when still soluble, and the lack of processability when mechanically strong. Various chemical and physical means have been applied to solve this issue, including using a post-cross-linking process. For instance, Andrei and coworkers (Solid State Ionics 72 (1994) 140–146), the contents of which are hereby incorporated by reference in its entirety, prepared a comb branched copolymer incorporating allyl functional groups at the end of the side chains. These allyl groups were allowed to undergo a hydrosilation reaction with excess triethoxysilane in the presence of H2PtCl6H2O catalyst. The copolymer now carrying triethoxysilyl groups was dissolved, along with lithium salt and acidified diethylene glycol, in a solvent and was cured in the shape of a thin membrane. Transparent and easily handled membranes were obtained with good mechanical properties and no penalty due to the cross-linking in terms of conductivity. While the post-cross-linking strategy is appealing, there are some problems with this prior art polymeric electrolyte design:
1) The concentration of allyl groups in the polymer is very low, which means an excess amount of the other reactant has to be used in order to obtain acceptable conversion of the allyl groups. After the completion of hydrosilation, the excess silane is now an impurity and should be removed as completely as possible. However, triethoxysilane employed has a relative high boiling point (134° C.), which makes it impossible to completely remove the excess. This excess silane will affect the electrochemical properties of the electrolyte.
2) Since the mechanism of the cross-linking involves the breaking and reformation of Si—O—C bonds, the chemical stability of this network is in question. These linkages are subject to hydrolysis due to moisture. Any unreacted, acidified diethylene glycol will mean introduction of hydroxyl groups in the membrane and would react with lithium metal in a cell.
3) The catalyst used contains trace amounts of water that should be avoided.